This invention relates to an ultrasound transducer and a method for harmonic imaging, and in particular, to an ultrasound transducer and method for focusing an ultrasound beam at multiple focal points and utilizing reflected ultrasonic energy at a harmonic of a transmitted fundamental frequency signal, for example, to image contrast agents or for tissue harmonic imaging when no contrast agent is used.
Contrast agents are described, for example, by V. Uhlendorf, et al., in "Nonlinear Acoustical Response of Coated Microbubbles in Diagnostic Ultrasound" (1995 Ultrasonic Symposium, pp.1559-1562). Such contrast agents possess a fundamental resonant frequency. When they are insonified with high intensity ultrasonic energy at this fundamental frequency, a strong non-linear response is caused which is stronger in the agent than in the tissue, i.e., they radiate ultrasonic frequency at a harmonic of the fundamental frequency. Such contrast agents are often used to highlight regions containing blood loaded with the contrast agent. For example, in the case of a blood-filled chamber of the heart, the borders of the chamber can be distinguished more easily when contrast agent is used. Since the contrast agent generates more harmonic ultrasound energy, than from tissue signals, signals from the tissue may be reduced by filtering out the fundamental response at the receive beamformer. In addition, it may be desirable to provide an ultrasonic system that is responsive to reflected harmonic signals even when a contrast agent is not injected into a target because tissue itself produces a non-linear response.
Typically, contrast imaging is performed with an imaging system having a transmit beamformer that transmits ultrasonic energy at a fundamental frequency and a receive beamformer responsive to a harmonic of the fundamental frequency. In order to image the contrast agent or tissue, if no contrast agent is used, clearly, it is known to reduce energy at the harmonic in the transmit beam, and to reduce sensitivity of the receive beamformer to energy at the fundamental frequency. In the past, this has been accomplished by using a burst of square or sine waves to form the transmit beam, and by using appropriate band pass or high pass filters in the receive beamformer. Though a large pulse count reduces energy at the harmonic, it reduces time resolution of the pulse, and therefore spatial resolution of the resulting image.
The present invention is directed to further improvements that enhance the collection of reflected ultrasonic energy at a harmonic of a transmitted signal.